The present invention proposes a method for diagnosing the function of an oxidation catalyst, for the conversion of NO into NO2, during operation on board a vehicle, which oxidation catalyst is arranged in a vehicle containing an internal combustion engine which, during operation, emits exhaust gases to an exhaust gas aftertreatment system containing the aforementioned oxidation catalyst.
The present invention also proposes an engine-driven vehicle containing an internal combustion engine which, during operation, emits exhaust gases to an exhaust gas aftertreatment system containing an oxidation catalyst for the oxidation of NO to NO2, an injection device for injection of a reducing agent into the exhaust gas aftertreatment system upstream of the oxidation catalyst, an NOx-reducing catalyst and/or particulate filter arranged downstream of the oxidation catalyst, a gas sensor arranged at least downstream of the oxidation catalyst and a control unit for recording signals from the gas sensor and for controlling at least the injection device.
The present invention also proposes a computer program product containing a computer program intended to execute such a method with a computer.
Statutory requirements relating to diesel engines have been tightened and will be tightened further, in particular with regard to emissions of nitrogen oxide compounds and particulates. The quantity of oxides of nitrogen formed by the combustion of fuel in the cylinder of an engine is dependent on the temperature during combustion. Higher temperatures lead to the conversion of a larger proportion of the nitrogen present in the air into oxides of nitrogen. The catalysts that are used on diesel engines and other engines which operate with an excess of air are for the most part oxidizing catalysts. Because the exhaust gases contain oxygen, it is difficult to reduce the oxides of nitrogen with high selectivity. In addition to oxides of nitrogen, the undesired emissions carbon monoxide (CO), hydrocarbons (HC) and particulates inter alia are also formed during the combustion process primarily in the form of soot (C).
A previously disclosed method for reducing the quantity of oxides of nitrogen, and which is based on exhaust gas aftertreatment, is the LNA (Lean NOx Adsorber) NOx adsorber. LNA can also be referred to as LNT (Lean NOx Trap). The method is based on first oxidizing NO into NO2 in an oxidation catalyst, after which the NO2 is stored in the adsorber in the form of nitrates. The storage of NO2 occurs when the engine is operating with an oxygen surplus. Regeneration of the NOx adsorber (NOx-reducing catalyst) then occurs intermittently at predetermined intervals by causing the engine to operate with an oxygen deficiency, that is to say, with the addition of extra hydrocarbon (a reducing agent) and/or a reduced air flow, which destabilizes the nitrates and reduces the nitrogen dioxide NO2 trapped in the NOx adsorber into nitrogen N2 and water H2O. See, for example, U.S. Pat. No. 5,473,887 or U.S. Pat. No. 6,718,757. Both the storage and the regeneration require the temperature in the NO: adsorber to be sufficiently high (more than 200° C. for storage and circa 300° C. for regeneration). At low loadings on the engine (e.g. urban driving or an unladed goods vehicle), the exhaust gas temperature will not be sufficient to maintain the NOx adsorber at the necessary temperature. One way of forcing the temperature up to the appropriate level is to inject hydrocarbons into the exhaust gas that is then burned catalytically in the NOx adsorber so that the right temperature is achieved. The hydrocarbons have a negative influence on the useful NO2 formation, whereupon the total conversion of oxides of nitrogen in the exhaust gas system decreases during heating up. In accordance with the prior art, it is possible to control the injection in such a way that the hydrocarbon to all intents and purposes poisons the oxidation catalyst totally so that the formation of NO2 in the oxidation catalyst is in principle non-existent.
If the oxidation catalyst for some reason has an impaired NO2 formation function, a reduced quantity of NO2 will be stored in the NOx adsorber and an increased quantity of NOx will be released into the atmosphere.
In conjunction with the supply of hydrocarbon, this can take place as an extra injection (post-injection) with an exhaust valve open in the engine or via an injector arranged on the exhaust pipe.
Another previously disclosed exhaust gas aftertreatment method, to which the formation of NO2 through an oxidation catalyst is central, is CRT (Contmously Regenerating Trap). Particulates, that is to say soot and sulfur compounds, for example, are collected here in a trap, where the soot can be transformed into carbon dioxide CO2. The NO2 functions here as an oxidation agent in conjunction with the conversion of the particulates. In order to ensure that the soot combustion takes place with the help of NO2, the temperature of the exhaust gas aftertreatment system needs to be above 250° C. Here, too, the temperature in the exhaust gas aftertreatment system can be increased to an appropriate level with the help of the addition of hydrocarbons that are burnt in the catalyst.
If the oxidation catalyst in the CRT for some reason has an impaired NO2 formation function, a reduced quantity of soot will be oxidized in the particulate filter, which means that there is a risk that the particulate filter may become overcharged and that a sufficiently high temperature in the particulate filter can give rise to a soot fire, which, thanks to the increased quantity of soot that is burnt, can develop to such an extent that the particulate filter can be damaged.
Other previously disclosed exhaust gas aftertreatment techniques to which the formation of NO2 is central are:                LNC (Lean NOx Catalyst), in which oxides of nitrogen are reduced continuously under oxygen-rich conditions.        Particulate filters coated with “washcoat”.        Urea or ammonia-based SCR (Selective Catalyst Reduction) for NOx reduction; see, for example, U.S. Pat. No. 5,540,047.        Hydrocarbon-based (HC-based) SCR (Selective Catalyst Reduction).        
In order to guarantee the function and, consequently, that the statutory requirements are met, various diagnoses are performed on board and during operation of the vehicle on parts of or on the entire exhaust gas aftertreatment system. EP1174601 illustrates an example of a diagnostics method for an exhaust gas aftertreatment system based on temperature measurements. A predetermined quantity of hydrocarbon HC is injected periodically. The exotherm is measured with a temperature sensor, that is to say a recording is made of the light-off temperature, and on the basis of the measured temperature values a decision is taken in respect of whether or not the exhaust gas aftertreatment system has an impaired function.
It is desirable to diagnose the oxidation catalyst and its formation of NO2 on board and during operation of the vehicle, so that any malfunction can be identified in good time and any undesired exhaust gas emissions can be reduced in this way.
The method in accordance with the invention includes a diagnostics method performed during operation on board a vehicle and for an oxidation catalyst, for the oxidation of NO into NO2, arranged in a vehicle with an internal combustion engine which, during operation, emits exhaust gases into an exhaust gas aftertreatment system containing at least the aforementioned oxidation catalyst and an NOx-reducing catalyst arranged downstream of the oxidation catalyst, and a predetermined quantity of a reducing agent is supplied to the exhaust gas aftertreatment system during at least a predetermined time interval upstream of the oxidation catalyst, and the NOx content is measured downstream of the NOx-reducing catalyst. The method is characterized by the following stages:                a first measurement and recording are made of a first value for the NOx content at a point in time immediately before the reduction agent is supplied to the exhaust gas aftertreatment system;        a second measurement and recording are made of a second value for the NOx content at a point in time during the period when the reduction agent is supplied to the exhaust gas aftertreatment system and poisons the aforementioned oxidation catalyst;        a comparison is made between the aforementioned first and second values, in conjunction with which an indication of the impaired function of the oxidation catalyst is obtained when the difference between the aforementioned measured values is less than a predetermined first value.        
An advantage that is obtained with the method in accordance with the invention is that oxidation of the catalyst function can be diagnosed continuously when the vehicle is in operation, and an indication is obtained in the event that oxidation of the catalyst function is impaired. A more stable exhaust gas aftertreatment function is obtained in this way, and a minimization of undesired exhaust gas emissions can be assured.
The invention also includes a device in the form of an engine-driven vehicle with an exhaust gas aftertreatment system in which the oxidation catalyst is diagnosed in accordance with the present invention.
The advantages achieved with the device in accordance with the invention are the same as with the method in accordance with the invention.
In an alternative embodiment of the method and the device in accordance with the invention, the difference is also compared with a second predetermined value, where the aforementioned second predetermined value corresponds to a limit for a statutory maximum permissible exhaust gas emission. If the difference is greater than the aforementioned second predetermined value and less than the aforementioned first predetermined value, the aforementioned indication will take place in conjunction with the vehicle's next regular service.
One advantage of this is that the flow of information to the vehicle's driver is reduced, and that workshop inspection and any repair of the exhaust gas aftertreatment system can take place in a more cost-effective way.
In another alternative embodiment of the method and the device in accordance with the invention, a particulate filter is arranged downstream of the oxidation catalyst, in conjunction with which the difference is instead compared with a third predetermined value. If the difference is less than this predetermined value, an indication of the impaired function of the oxidation catalyst is given immediately to the driver of the vehicle. This is because of the rapidly increasing risk of a future soot fire damaging the particulate filter.
In a further alternative embodiment of the method and the device in accordance with the invention, the NO2 content is measured instead with the help of a gas sensor arranged downstream of the oxidation catalyst. Measuring the NO2 content with a gas sensor is in itself previously disclosed. The characterizing stages are the same as in the corresponding embodiment referred to above, except that the NO2 content is measured instead:                a first measurement and recording of a first value for the NO2 content are made at a point in time immediately before the reduction agent is supplied to the exhaust gas aftertreatment system;        a second measurement and recording of a second value for the NO2 content are made at a point in time during the period when the reduction agent is supplied to the exhaust gas aftertreatment system and poisons the aforementioned oxidation catalyst;        a comparison is made between the aforementioned first and second values, in conjunction with which an indication of the impaired function of the oxidation catalyst is obtained when the difference between the aforementioned measured values is less than a predetermined first value.        
The principal advantage is the same as for the corresponding embodiment described above.